Interest in the separation of carbon dioxide and hydrocarbons in gas streams containing a large percentage of carbon dioxide comes primarily from two areas: (1) recovery of hydrocarbons from gas streams which were previously thought uneconomical to recover, and (2) recovery of carbon dioxide from gas streams associated with enhanced oil recovery (EOR) projects employing carbon dioxide for miscible flood of oil reservoirs, thereby allowing the carbon dioxide to be reinjected into the reservoir.
Gas permeable membranes have found high volume industrial applications including gas-mixture separation, such as removal of carbon dioxide from natural gas. Other processes such as cryogenic distillation processes have been developed for separating acid gases such as carbon dioxide and hydrogen sulfide from hydrocarbons.
Although stand alone membrane separations systems and stand alone cryogenic distillation systems each provide some latitude for processing gas streams, there are inherent economic limitations to the usefulness of each technique. For example, to achieve a high purity separation in a stand alone membrane process it is generally necessary to stage multiple membrane units. In this membrane process the permeate from the first stage is compressed and fed to the second stage, thereby necessitating costly recompression of the interstage stream. Stand alone distillation processes for gaseous streams also have economic limitations. Generally distillation requires cryogenic processing for the condensation of impurities, and is considered to be economical on-y for processing gas streams having high CO.sub.2 levels, and where the cryogenic unit does not have to produce the final product and can therefore operate at a lower purity.
Since distillation techniques for separating carbon dioxide and light hydrocarbons are generally economical for processing high carbon dioxide content gas streams, and membrane techniques are generally economical where a single stage membrane separation is employed, a separation system which includes distillation plus membrane techniques while avoiding staging of multiple membranes and cryogenic distillation of low carbon dioxide content gas streams would be highly desirable.
It is therefore an object of this invention to provide an improved method for separating carbon dioxide from light hydrocarbons in a mixture predominating in carbon dioxide.
It is a still further object of this invention to provide method and apparatus for the separation of carbon dioxide from light hydrocarbons which is effective, efficient, safe, and economical.
It is yet another object of this invention to recover a carbon dioxide stream from an EOR project which is substantially free of impurities such as methane so as to be suitable for reinjection into an oil reservoir.